This invention relates to improved chromated copper arsenate wood treating compositions. More particularly, this invention is directed to chromated copper arsenate wood treating solutions in which the ratio of hexavalent chromium (Cr+6) to trivalent chromium (Cr+3) is between about 4.0:1.0 to about 1.0:1.0. The chromated copper arsenate solutions having ratios of Cr+6 to Cr+3 within this range provide improved penetration into the wood, better retention of preservative in the wood, and improved stability of the treating solutions. The solutions also contain an acid. The acid is selected to maintain the desired pH while further improving solution stability and preventing corrosion. This invention also relates to the treatment of wood with the improved compositions to prevent decay, fungi, rot, termites and the like.
During recent years the group of wood preservatives known as chromated copper arsenates (CCA) have become of great importance. These CCA compositions are one of the most widely used type of wood preserving compounds in use today. These preservatives contain as active ingredients copper as cupric ion, chromium usually as the dichromate ion and arsenic as acid arsenate ion. In addition, the CCA compositions may also contain inert ions and salts such as sodium ion, sulfate ion, and/or sodium sulfate. These inert ions and salts are the side products which are formed when the CCA compositions are formulated from the least expensive source of active ingredients usually available; for example, copper sulfate as the copper source and sodium dichromate as the chromium source. Generally the compositions contain from about 40.0 to about 70.0% by weight chromium measured as CrO.sub.3, from about 15 to about 23% by weight copper measured as CuO, and from about 14 to about 50% by weight arsenic measured As.sub.2 O.sub.5.
Typical formulations of some of the widely used CCA compositions of the prior art are as follows: Greensalt which is 56% Na.sub.2 Cr.sub.2 O.sub.7 . 2H.sub.2 O or K.sub.2 Cr.sub.2 O.sub.7, 33% CuSO.sub.4 . 5H.sub.2 O, 11% AS.sub.2 O.sub.5 . 2H.sub.2 O; Tanalith C which is 45% Na.sub.2 Cr.sub.2 O.sub.7 . 2H.sub.2 O or K.sub.2 Cr.sub.2 O.sub.7, 35% CuSO.sub.4 . 5H.sub.2 O, 20% As.sub.2 O.sub.5 . 2H.sub.2 O; and Celcure A which is 40% Na.sub.2 Cr.sub.2 O.sub.7 .2H.sub.2 O or K.sub.2 Cr.sub.2 O.sub.7, 32% CuSO.sub.4.5 H.sub.2 O, 28% As.sub.2 O.sub.5.2H.sub.2 O. In recent years, the American Wood Preservers Association (AWPA) has standardized the formulations of CCA compositions. The formulations have been converted for greater ease in specification into a uniform nomenclature based on the oxides CrO.sub.3, CuO and As.sub.2 O.sub.5. The currently adapted standardized American formulations have the following compositions in percent by weight:
CCA-Type A CCA-Type B CCA-Type C ______________________________________ CrO.sub.3 % 65.5 35.3 47.5 CuO% 18.1 19.6 18.5 As.sub.2 O.sub.5 % 16.4 45.1 34.0 ______________________________________
The formulations are prepared and sold as concentrated solutions containing from about 45 to 75% active oxides in water. The concentrated solutions are then diluted to about 1 to 10% active oxides for use in treating wood.
Several patents have issued on the use of chromated copper arsenate wood preservative compositions. See for example, Hager, U.S. Pat. No. 2,202,579; 2,366,612; 2,432,007, and 2,565,175. In addition see McMahon, U.S. Pat. No. 2,438,511, Kamesam, U.S. Pat. No. 2,106,978, Oberley et al, U.S. Pat. No. 3,080,212, Henriksson et al, U.S. Pat. No. 3,793,441 and Nicholson, U.S. Pat. No. 3,832,463. All of these patents disclose the use of chromated copper arsenate compositions which are useful in preserving wood. However, all of the compositions contain the chromium only in the hexavalent state.
Chromated copper arsenate solutions are particularly effective preservatives for the prevention of decay and deterioration of wood,. Wood impregnated with chromated copper arsenate solutions are resistant to decay and attack by termites and at the same time the active ingredients are resistant to weathering and leaching when wood treated with the compositions is in contact with the ground or water. Moreover, the treated wood does not bloom and hence the surface of the treated wood may be painted.
Heretofore, wood was commonly treated with chromated copper arsenate solutions in which the chromium was present in the hexavalent form. Treatment with these prior art chromated copper arsenate solutions had several drawbacks. First, there is incomplete penetration of the chromated copper arsenate solutions into the wood. This is particularly true of refractory species of wood such as Douglas fir. In addition, the initial retention of the chromated copper arsenate compositions is not as high as is desirable. Another drawback of the prior art CCA treating solutions is their instability in the presence of reducing sugars normally found in most untreated woods. The CCA solutions react with the reducing sugars during treatment to form sludgy precipitates. The precipitates interfere with penetration and retention, greatly reduces the usable life of the treating solution and causes corrosion of the treating equipment. This results in a large chemical and monetary loss.
It is, therefore, the object of this invention to provide an improved chromated copper arsenate wood treating solution which overcomes the drawbacks of the prior art chromated copper arsenate solutions by providing improved penetration, retention, and solution stability.